Actuator devices of the above mentioned type typically include a solid state electrostrictive or piezoelectric laminated element as an actuator, which becomes elongated in a lengthwise direction and which contracts in a crosswise or transverse direction, upon application of an electrical voltage to the solid state element. This description will generally refer to piezoelectric materials for the actuator element as an example, but applies equally to electrostrictive materials.
Such actuator devices are characterized by a relatively high actuating speed, a high force application, and a high resolution. However, since such solid state actuators are extremely sensitive to tensile loading, i.e. they have a relatively low effective tensile strength, it has been found that they should be arranged in such a manner that the solid state element is pre-compressed in the lengthwise direction by a pre-loading spring. This is especially the case in applications requiring rapid actuation switching.
Furthermore, the respective end faces of the solid state (e.g. piezoelectric) laminated actuator element are coupled to the other components of the device, such as the actuator pin and the housing or frame, so as to transmit the elongation force and travel from the actuator element to the actuator pin relative to the housing. Such coupling, if direct, causes a problem as follows. The tendency of the solid state laminated actuator element to contract in a transverse direction upon being energized is constrained at its end faces, due to the coupling of the end faces to the actuator pin and housing, which do not contract. If no protective measures are provided, the transverse constraining effect results in excessive transverse tensile loads and shear stresses in the laminated actuator element, near the lengthwise ends thereof.
One attempt to avoid the above mentioned transverse tensile stresses has been to arrange a protective interlayer of homogeneous soft elastic material between each end face of the laminated actuator element and the adjoining solid component of the device. In this manner, the coupling between the actuator element and the solid components of the device is intended to be somewhat yielding in the transverse direction, in order to provide protection against excessive transverse tensile loads arising at the end faces of the actuator element due to the transverse contraction thereof. However, such an elastic interlayer also reduces the mechanical stiffness and positive coupling of the force transmission chain, so that at least a portion of the lengthwise piezoelectric elongation is lost or wasted in the compression or deformation of the soft elastic material of the interlayer, and only a limited portion of the piezoelectric elongation is available for the useful actuation motion. This is especially true because the piezoelectric elongation is typically characterized by a high force and relatively short travel.
Japanese Patent Publication 1-226186 describes another attempt to overcome the above mentioned problem of transverse tensile loading and resulting shear stresses. According to this reference, the outermost layers of the laminated piezoelectric element are made of a piezoelectric material that has a smaller transverse voltage strain constant than does the piezoelectric material of the rest of the laminated piezoelectric element. Thereby, the outermost piezoelectric layers act as stress moderating layers, because they will tend to exhibit a contraction strain that is less than, and particularly only one half of, the transverse contraction of the rest of the laminated piezoelectric element. However, the end faces of the laminated piezoelectric element apparently remain rigidly coupled to the rest of the actuator device, e.g. the actuator pin and the housing. Thus, the arrangement according to Japanese Patent Publication 1-226186 still suffers an abrupt peak in the transverse tension at the rigid or fixed junction between the piezoelectric element and the rest of the actuator device, which results in undesirably high transverse tensile loads on the end faces forming the junction surfaces of the outermost piezoelectric layers.